1. Field of the Invention
The present invention relates to a method of measuring the strength of concrete for the slip form process and, more specifically, to a method of measuring the strength of concrete on the basis of an integral of concrete temperature as a function of time, as concrete pouring progresses.
2. Description of the Prior Art
As is generally known, the slip form process is applied extensively to building tower concrete structures such as elevated tanks, tall storage bins and silos.
In the slip form process, relatively short form panels are assembled side by side to construct a slide form. Concrete is progressively poured in the slide form while the slide form is moved upward slowly by means of rods buried in the poured concrete which, when hardened, consititues the wall of the stucture. Naturally, the concrete already poured must have a sufficient strength to support itself and the slide form to allow the slide form to be moved upward. The strength of concrete increases gradually with the lapse of time after pouring. The rate of increase is dependent on the ambient conditions, such as the atmospheric temperature, himidity and weather conditions. Accordingly, in the slip form process, it is necessary to measure the strength of the concrete poured in the slide form as concrete pouring progresses in a manner, for example, as disclosed in Japanese Patent Publication No. 52-16336.
According to a measuring method disclosed in the above-mentioned Japanese patent publication, a pressing member is pressed against the concrete poured in the slide form to determine the strength of concrete on the basis of the relation between the stress that acts on the pressing member and the amount of displacement of the pressing member under the pressure applied thereto. However, this method involves disadvantages in that depressions tend to be formed and remain in the surface of the concrete structure by the pressing member, and that it is impossible to measure the strength during elevational movement of the slide form.
Japanese Patent Laid-Open No. 59-118961 discloses an automatic control of elevational movement of the slide form on the basis of measured strength of concrete poured in the slide form. The strength is measured by inserting a sensor into the concrete, and a signal from the sensor is forwarded to a comparator to determine whether the strength is sufficient in view of a predetermined value. This prior art, however, does not suggest how the sensor can measure the strength.
Further, Japanese Patent Laid-Open No. 60-199164 shows a method of measuring strength by inserting a rod into the poured concrete. The rod is inserted under a predetermined stress, and an amount of displacement of the rod indicates a level below which the concrete is considered to have a sufficient strength. This method requires that the stress applied to the rod must exactly be set to a predetermined value so as to avoid destruction of the hardened concrete, however, this is practically difficult.